1. Field of the Invention
The invention relates to a physiological sensor system for recording electrical measurement signals in an environment which impairs this recording, particularly in a magnetic resonance instrument, having a plurality of measurement electrodes as well as a signal amplifier device, a power supply, and an electronic device for signal conversion and transmission to an external signal processing and/or control instrument.
Such a physiological sensor system is used for the in situ recording of physiological measurement values, for example, during an examination of a patient using a magnetic resonance instrument. By using such a sensor system, it is, for example, possible to record an ECG during the examination so that, on the one hand, the heart activity can be registered continuously and, on the other hand, the imaging of the magnetic resonance instrument can be controlled by continuously registering the heart""s position. If the magnetic resonance images show, for example, the heart in a certain valve position, then the moment at which the heart is in the desired valve position can be registered exactly through the ECG signals, and the imaging can be triggered as a result of this information.
2. Description of the Related Art
Such a physiological sensor system is known from U.S. Pat. Nos. 5,782,241 and 6,052,614. In this sensor system, there are a number of measurement electrodes which are applied directly to the patient""s skin. The electrodes are arranged at the lower end of a shielded casing. The casing further contains radio frequency filter devices to each of which an electrode is allocated, as well as a differential amplifier unit, a low pass filter, an electro-optical transducer for converting the measurement signals into optical signals that are given through an optical data line to an external processing and display device, as well as a power source in the form of a battery.
All of the elements relevant to the operations of recording and preprocessing the measurement signals are hence arranged together in the casing, which is applied to the patient. However, this is disadvantageous because of the considerable structural size and the simultaneous integration of the measurement electrodesxe2x80x94the sensor system has to be positioned close to the heart. Moreover, this entails the risk that this sensor system lies at least partially in the imaging region, i.e., in the region from which the magnetic resonance image is intended to be recorded. The latter is thereby at least impaired.
European patent document EP 0 173 130 A1 discloses a device for nuclear spin tomography, in which the electrodes are connected through a cable link to an amplifier device located externally to the nuclear spin tomography device. From this amplifier device, which is arranged together with the nuclear spin tomography device in an RF cabin, the measurement signals are given through an optical waveguide with a connection to a processing device located externally to the cabin. German patent document DE 33 27 731 A1 describes a device for obtaining an ECG signal in a nuclear spin tomograph, in which the nuclear spin tomograph is likewise arranged in an RF cabin, the recorded signals being fed through a shielded connection, which is set to the electrical potential of the RF cabin, in order to avoid interference with the NMR image. German patent document DE 198 17 094 describes a method and a device for deriving an electroencephalogram in the nuclear spin tomograph, while U.S. Pat. No. 4,737,712 describes an isolated power source which can operate in a strong magnetic field and an RF field, as may be found, e.g., in an NMR instrument. Lastly, U.S. Pat. No. 5,052,398 describes a filter suitable to be used in an NMR instrument for real-time heart representation, while German patent document DE 41 38 702 A1 describes a method and a device for diagnosis and quantitative analysis of apnoea and for simultaneous identification of other diseases. Lastly, German patent document DE 41 23 578 A1 describes a non-invasive method for spatial registering of local heart potentials.
It is an object of the invention to provide a physiological sensor system which affects the imaging as little as possible, yet allows minimally distorted recording of the measurement signals.
To achieve this object, in a physiological sensor system of the type mentioned previously, according to the invention, the measurement electrodes and the signal amplifier device are arranged in or on a shielded first sensor casing to be arranged close to the patient, and the power supply and the electronic device are arranged in a second shielded casing to be arranged close to the patient, the signal amplifier device being connected or connectable to the electronic device and the power supply through a shielded and/or twisted-wire cable connection.
The sensor system according to the invention comprises two shielded casings, which are constructed in the manner of a Faraday cage, and which contain the components needed for the measurement value recording and pre-processing. Only the measurement electrodes and the signal amplifier device are present in the shielded first sensor casing. This first sensor casing is applied directly to the patient, in the region near the heart in the case of recording an ECG. Since only the measurement electrodesxe2x80x94generally threexe2x80x94and the signal amplifier device are integrated in this sensor casing, it is very small so that it can be positioned in such a way that exact measurement value recording is possible, although because of its size it does not substantially affect the imaging.
The recorded measurement signals are given through a shielded or twisted-wire cable connection to the second shielded casing, and there to the electronic device and the power supply. The shielded or twisted-wire cable connection ensures that the analog and amplified measurement signals can be transmitted substantially unaffected by the strong magnetic fields which exist during the operation of a magnetic resonance instrument. This means that the signal-to-noise ratio is virtually unchanged. The second casing can then be positioned fully out of the image-relevant examination region. The length of the cable connection should expediently be in the range between 20-30 cm, although it may be greater.
The signal amplifier device should expediently be arranged in immediate proximity to the measurement electrodes so as to minimize the admission of noise signals. If a plurality of measurement electrodes are arranged in the sensor casing, then a common signal amplifier device can be allocated to them. Alternatively, each measurement electrode can be provided with its own amplifier.
Besides a system configuration having one first sensor casing and one second shielded casing, it is also possible for the system to comprise a plurality of sensor casings, each with a plurality of measurement electrodes and an allocated signal amplifier device, different measurement signals being recordable by the measurement electrodes of a respective sensor casing, and each signal amplifier device being connected or connectable to the common electronic device and the common power supply through a separate shielded or twisted-wire cable connection. In this multifunctional sensor system, for example, one first sensor casing can be used to record ECG measurement signals and the other sensor casing can be used to record EEG measurement signals. Both are very small, since they only contain the electrodes and the amplifier device, and the common signal preparation takes place in the common second shielded casing.
It is particularly expedient for the signal amplifier device, in the case of measurement electrodes intended to record ECG measurement signals, to be designed just to amplify the measurement signals. As an alternative to this, it is possible for the amplifier device also to be designed to form leadoff-specific differential signals. In the scope of an ECG measurement, which corresponds to the extremity leadoffs according to Einthoven, three measurement electrodes are used which are oriented in different directions. A first measurement electrode goes to the left arm, a second to the right arm and a third to the left leg. Differential signals between two measurement electrodes are respectively recorded as ECG measurement signals. Each differential signal corresponds to one leadoff, three leadoffs being possible in all, namely, the first differential signal of the measurement electrodes xe2x80x9cleft arm-right armxe2x80x9d, the second differential signal xe2x80x9cleft leg-right armxe2x80x9d and the third differential signal xe2x80x9cleft leg-left armxe2x80x9d. For technical processing reasons, it is expedient if this difference formation is already carried out on the part of the amplifier device. Operational amplifiers are expediently used for the amplification and/or differential signal formation.
The electronic device itself expediently has a signal conversion module and a signal transmission module, both of which are coupled to the power supply. The term xe2x80x9cmodulexe2x80x9d is here used very generally to denote a circuit arrangement with the smallest possible dimensioning, which is used for either the signal conversion or the signal transmission. All the elements needed for this can be arranged on a common board. Since, naturally, it is also desirable for the second shielded casing to be dimensioned as small as possible, common integration of the modules is expedient.
The signal conversion module should expediently have at least one filter and at least one converter unit. An analog/digital converter or a voltage/frequency converter can be used as converters.
If a plurality of measurement signal inputs are provided at the second shielded casing, or at the electronic device, then it is expedient for a separate filter to be provided for each measurement input, the filters being connected to the converter through a multiplexer. A filter is expediently provided for each measurement signal input, irrespective of the type of the measurement signals which are delivered, i.e., whether they are, for example, ECG or EEG measurement signals which can be registered together and read out in multiplex operation. The operation of the multiplexer can expediently be controlled externally through a control line, so that the signal recording can be controlled externally through an external signal processing and/or control instrument.
The power supply can have a battery, or an accumulator to which a switch that can be actuated through an external magnetic field is allocated. This configuration is expedient so long as, during operation of the magnetic resonance instrument, a sufficient magnetic field is applied so that the switch is actuated and the supply circuit is closed. As soon as the sensor system has been removed from the magnetic resonance instrument, the switch re-opens and the power supply is hence not kept continuously on.
As an alternative to this, the power supply can also contain one or more solar cells which can be illuminated with light that can be fed through an optical fiber, particularly laser light. According to a third alternative of the invention, the power supply contains one or more solar cells which can be illuminated with light that can be fed through a fluorescence collector, which is arranged on the outside of the casing and captures ambient light. Such a fluorescence collector may be a plate material made of polymethyl methacrylate, into which fluorescein and other fluoropolymers are polymerized. This collector gathers incident diffuse ambient light, which is concentrated on its end face and emerges through a narrow region. According to the invention, this concentrated light strikes the solar cell configuration which in turn generates energy.
Lastly, according to a fourth alternative of the invention, the power supply contains at least one capacitor which is connected to coils that are arranged on the outside of the casing and deliver a voltage when an external magnetic field is applied. This configuration according to the invention utilizes the magnetic field which is applied anyway during operation of the magnetic resonance instrument, by the fact that, on the outside of the casing, coils are arranged in which the magnetic field leads to the induction of an AC voltage, which is used after rectifying to charge a capacitor that then supplies the individual system elements with power.
It has been found to be particularly advantageous if at least one storage capacitor, having a storage capacitancexe2x89xa71 F, which provides power during operation, is allocated to the solar cells. This storage capacitor which, in the field, is known as xe2x80x9cUltra Capxe2x80x9d or xe2x80x9cGold Capxe2x80x9d, has a relatively high capacitance and can hence store a sufficient amount of power, which it then gives out during operation. This ensures that the system elements are only supplied with voltage when the power delivered by the solar cells, or by the at least one capacitor connected to the coils, is not sufficient. The storage capacitor can expediently be recharged in an external charging station. When solar cells are used, the charging station would generate maximally energetic light in order to guarantee a short charging time.
The signal transmission module can, according to a first configuration of the invention, be a radio transmission module, with which the recorded, amplified and subsequently converted measurement signals can be transmitted wirelessly to the external processing device.
According to an alternative embodiment, however, the signal transmission module may be an infrared transmission module having at least one infrared transmission diode with an allocated optical line that leads from the casing but not to the receiver, in which case a plurality of diodes, in particular three, to each of which an optical line is allocated, may expediently be provided.
According to a third configuration of the invention, the signal transmission module can be a fiber optic transmission module having at least one transmission diode with an allocated fiber optic line. Lastly, a fourth alternative embodiment provides that the signal transmission module be an ultrasonic wave module having at least one ultrasound transducer.
It is expedient for the multiplexer to be externally controllable. To that end, an input for the external control line can be provided at the signal transmission module, the control signals being looped through internal control lines from the signal transmission module to the signal conversion module, optionally through the power supply. In this case, it is expedient for an optical receiver, which transforms the optical control signals provided through the optical control line into electrical control signals, to be allocated to the internal control line provided at the signal transmission module. According to a second alternative, in the case of a configuration of the signal transmission module as an ultrasonic wave module, the ultrasound signal line is also used to transmit the control signals, so that it is used simultaneously as a forward and return line. The control signals are transformed by the ultrasound transducer into electrical control signals, which are then forwarded through the internal control line.
As described, it is possible to provide a plurality of sensor casings for recording different electrical measurement signals in the sensor system according to the invention. In order to increase the multifunctionality of the sensor system yet further, it has been found to be expedient that at least one further sensor element, which registers non-electrical measurement information, also be connected or can be connected to the second shielded casing, the electronic device having a corresponding mechanism for converting the non-electrical measurement information into electrical measurement signals. As a further sensor element, it is possible to provide an optical sensor element, particularly a finger ring which, for example, is placed around a patient""s finger and measures the patient""s peripheral pulse by transmitted illumination and absorption measurement. This optical sensor element delivers optical measurement information which is sent through a fiber optic line to the electronic device, where an optoelectronic converter for conversion into electrical measurement signals is provided. As an alternative, the at least one further sensor element can be a pneumatic sensor element, particularly a flexible chest ring, which is placed around the patient""s chest and by way of which the respiratory activity can be measured. This chest ring has a compressible air volume, which is coupled to the electronic device through a pressure line having a pressure sensor. Since, due to breathing, the air volume and therefore the pressure change, the pressure sensor experiences a continuously changing pressure, which can be converted into corresponding electrical measurement signals.